What are sim2sim, sim2real and real2real?
The abbreviations sim2sim, sim2real and real2real refer to techniques that can be used to transfer knowledge from one environment (e.g. in simulation) to another one (e.g. in the real world).


sim2sim stands for simulation-to-simulation,
sim2real stands for simulation-to-real, and 
real2real stands for real-to-real. 


In sim2sim, knowledge acquired during one simulation is transferred to an agent (or robot) in another simulation. Similarly, in sim2real, knowledge acquired during the simulation is used in a real-world problem (or environment). Finally, in real2real, knowledge acquired in a real-world problem can be transferred to another agent in another real-world problem.

The main challenges are related to the differences that exist between one environment and the other (either in simulation or in the real-world). For example, in sim2real, the simulation is almost never a perfect model of the real-world environment, so an agent trained in a simulation will probably not behave optimally in the real-world environment, which is often a lot more complex than the simulated environment. However, it is often the case that a robot needs to be trained in simulation, given that a robot trained in a real-world environment is subject to crashes.